I often get questions about why an Aux battery would be nice to have an
how I did mine, so I'll take this space to document what I did with my
installation.

I have a Main Bus, and a Critical Systems Bus (E-Bus), and a very
limited Aux Battery Bus. The architecture can be seen by looking
at my Power
Diagram.
It can be a little confusing to read what I ended up with, as it
doesn't directly mimic all of the popular Z-Diagrams, but instead pulls
in many aspects of them, and adds things I wanted. I have a bank
of 4 small 6v 12AH batteries in a series-parallel
arrangement to give 12V and 24AH in a thin but long pack. I
designed it this way to
fit in some commonly "wasted" space in the RV-10....under the flap
torque tube cover. The batteries are Werker
WKA6-12F (I'd recommend WKA6-12F2
instead because they're a .250" tab) from the local Batteries Plus store.
They're 5.94" x 1.97" x 3.74" and come in at 4.6lbs each. You
could do a bank of 2 or 2 banks of 4, depending on your load
wishes. Not only does this fill some wasted space, but it
keeps the Aux Batteries mounted very close to CG, and doesn't alter the
CG range as much as many other mounting options. If there's a drawback,
it's that you have to wire the battery pack since it isn't just one
single battery, but 4 of them. This can also be a big benefit,
because the 4 batteries can provide additional Ah capacity that might
have required a large battery to be mounted somewhere with more space,
like the tail...where it would affect CG greatly.

Here are some links to photos of the batteries. Follow the links
for more photos:

Many people design systems with
redundant alternators to provide power for systems should their main
alternator fail. Some people use single alternators with 2
batteries, some use dual alternators with dual batteries, and some use
dual alternators with single batteries...oh yeah, and some people even
use single alternators with single batteries. These things are
modular, in that you should be able to design the system to meet your
requirements. There may be a time when I'll add a standby
alternator, but right now I'm holding out until Plane-Power comes out
with one, which would be much cheaper than the common B&C offering.
For now, I find that I can get an hour's use or more out of my
Aux Batteries, if my alternator fails....and even more run-time if you
include the power left in the main battery. If my main battery
fries, I still have an Aux Battery to absorb the charge and keep the
power smooth, but I can shut off my Master switch and hopefully let
that Main battery be while I get on the ground. Either way, I feel it's
very nice to have some power redundancy since I do have a very
electrically dependent airplane. I also utilize Electronic
Ignition on one side, so having battery power is required to keep that
system alive, although I could limp home on a mag-only if I needed to.

Ground use for Pre-Flight Planning

When sitting on the ground preparing
for an IFR flight, you'll be doing things like contacting ATC for a
clearance, entering multiple waypoints/legs/airways into your FMS/GPS,
getting the local ATIS/AWOS, and in my case, getting your Datalink
Nexrad Weather picture of the current conditions. All of these
things require battery power. Sure, you can use the main battery,
but as many will find, you just don't want to sit there for 15 or 20
minutes (or more!) burning your one and only battery for those uses.
Having an Aux battery system allows you to use the power off that
battery as needed for these tasks, without even turning your master
switch on. (You use the Aux Battery Master)

Starting the Engine with al E.I.

When you crank the engine to start the
plane, your starter draws so
much current that the overall system voltage can draw down to 8V or
less. At some point, your EFIS and somes systems with specs of
10-32Vdc and such, will choose to glitch or reboot from the power drop.
To protect them, some people choose to power those systems off
during
engine start. Additionally, if you use Electronic Ignition, the power
drop can cause the system to misfire and cause engine kickback, which
can damage your starter. In fact, some starters are DESIGNED to break
from the kickback to protect the engine, and you'll be left sitting
there with no way to start your plane. Some E.I. manufacturers
recommend using an Aux battery to provide power to their system, even
if just for emergency and starting use, to provide a stable power
source during cranking. I have but one Battery Bus type item....my
Lightspeed Ignition. It's set up so that I can hit one switch and draw
E.I. power direct from the Aux Batteries, yet turn EVERYTHING else off.
This would be the absolute minimum power draw if I needed to keep
the plane in the air without avionics. I use this switch mode for
starting the engine. If I switch the switch OFF, it feeds the
Lightspeed it's power from the Main bus. You could just switch it
to power OFF, but it gives you more options to be able to feed it from
both places.

Regarding the EFIS specifically, some of them take a couple minutes to
boot up (less time while in-flight to keep the system-out time down to
a minumum if it knows you're flying), and having an Aux Battery can
allow you to power your EFIS on pre-start, and then LEAVE it on during
the engine start. This can be a great convenience item. The
more complex the EFIS, the longer the boot up process. I usually fire
up the EFIS and then start my walk-around, so when I get back to the
cockpit it's up and ready to go. I have a source select on my WSI
Wx receiver so that I can choose to power it from either the Avionics
Bus, or the E-Bus. That way I'm not wasting battery if I have an
emergency system, but if I want to have the convenience of Wx download
pre-start, I can do that. Another thing about the EFIS...there
are some systems with critical periods during the bootup process where
it's doing things that could leave you with corrupted software if the
EFIS were suddenly shut off. Having an Aux battery to keep things
stable can be a great help in preventing these kinds of issues.

One other starting-related item....your Engine Monitor. Many EFIS
systems display the engine parameters too. If you aren't able to keep
the system powered on during cranking, you'll never be able to see your
engine data such as Oil Pressure and Fuel Pressure during engine start.
Having an Aux battery can give you the ability to run that system
during engine start.

Maintenance use for Software Loading

I find that there is lots of time spent
playing in the hangar, loading software updates, NavData, and even
sitting in the cockpit making airplane noises. Again, during these
times, you can be saving your main battery by utilizing your Aux
battery for the power.

What goes on a "Critical Systems
Bus"?
So now that you see the benefits of an Aux Battery system, what would
you put on it? There are some very good schools of thought out there
that you should only put the absolutely necessary, minimum items on an
E-Bus, because by definition it's used for the best "ENDURANCE".
That's why I kind of avoid the term E-Bus. Yeah, I'm shooting for
endurance, but I also don't want to be limited in function unless I
HAVE to be. Fact is, I fly a 170kt aircraft, and I can absolutely find
an airport within 30 minutes of time if I lose my alternator. I
can easily be on the ground inside of an hour, IF I have my quality
instruments available. Without them, an IFR letdown could take a bit
more work and concentration and planning. So I choose to think of
it as a "Critical Systems Bus", and load it up a little heavier with
items that could be very handy in such a situation...especially since a
combined Main and Aux battery in my plane will power me well over an
hour and probably close to 2 hours or more, if the Alternator dies.

On my Critical Systems Bus, I have my PFD, my MFD, my AHRS, my
Autopilot, My SL-30 (primary Nav/Com), my EIS, my ADI, my Audio Panel,
and the ability to source select to get my WSI Wx if I'm not in a true
emergency. No lighting is on that bus, because I have very handy
red/white flashlights in highly accessible areas to the pilot, and I
have independent alkaline battery powered click-lights in the rear that
could be used in a pinch too.
Thinking about these items a bit:

PFD

The PFD could be your most important
instrument if you're in the clouds. You get attitude reference,
Navigation Info, the ability to switch it to an MFD and Engine monitor
format, and just wayyyy too many things to throw it away for the 3A
draw max that it has. And, it's on a pullable breaker so if you're
really in that bad of a spot, just pull the breaker.

MFD

Considering you can access MFD
functions on most (I have Cheltons) PFD's, this one is optional, but
you'd have to use good judgement if you had an emergency. Depending on
the situation, you may very much want the navigation and approach info
a MFD can provide. I left it on the Critical Bus because I can always
yank the breaker as I choose.

AHRS

This is a no-brainer if you have the
PFD/MFD there. They're useless without it. It's also on a
pullable breaker, so I can ditch it as necessary.

Autopilot

Again, if you're REALLY in a bad spot,
you need all the help you can get. If I know I'll be landing soon, I
may find that AP very helpful in getting down safely. It has it's own
panel mounted power switch, AND a pullable breaker, if I don't think I
need it, or know I can fly the approach without too much added stress.

SL-30

Pullable breaker, but hey, you want to
communicate, don't you? It switches off really easily with that little
round knob too.

EIS

Would be nice to analyze fuel
remaining, huh? Well, leave the EIS on the bus. It's on a
pullable breaker if you're really about to lose all power, and having
it alive will help keep your EFIS from barking at you regarding all of
your failed engine parameters if it were off.

ADI

Now here's a true minimum piece of
equipment. If all else fails, I can shut everything else off and use
the ADI. THAT is the worst case minimum power situation I want to be in
if I'm IFR, assuming that I'm also powering my Electronic
Ignition....which is not on the bus, but is switchable to run on the
Aux battery bus.

Audio Panel

Not the most necessary item if you're
in need of endurance, but considering the help a good co-pilot can be,
I'd rather get down quicker while communicating with a co-pilot than
have to yell without a headset to continue a flight that will only take
MORE time due to the complexity and lack of good communication.
Pullable breaker, so it's there.

WSI Wx

Convenience item only, so if I need to
run on minimum power, this gets turned off immediately. It's just on
the bus in case I want to get Wx before flight.

So, looking at the above, with the exception of the WSI, what's the
common thread? PULLALE BREAKER! Additionally, lots of that stuff
isn't very high power drawing anyway. It gives you the ability to shed
lots of other load quickly by flipping on the Critical Bus Alternate
Feed switch, and turning off the master, but doesn't leave you
handicapped at that instant...so you can keep your head on straight and
have what you need to make the quickest landing you can. I like the
E-Bus theory, but I'm not willing to toss out great safety tools
prematurely.

So the Aux battery system in my plane is compact enough, well placed
for CG and space concerns, gives plenty of power in an emergency, and
can prove highly useful for many reasons. What was the hardest part of
designing the system? Well, it went very well, but not perfectly. If
you study the Power
Diagram,
(it's as accurate as I could keep it), you'll see the general layout.
The odd feature you don't see often is the 2 Diodes. Many people use a
single E-Bus Diode for feeding their bus from the main for charging, to
prevent that bus from back-feeding the main if your main bus is off. I
have such a diode in my system. I also added a 2nd diode. I did use an
Avionics Master, which I really like for convenience of being able to
shut everything off fully and easily, and prevent mistakes that would
cause me to drain batteries. Not everyone likes an Avionics Master
though, but with my 2 diodes, I find no big downside. The 2nd diode in
the system basically allows the Critical Systems Bus items to stay live
while the switch is turned from the active to inactive state. To say it
differently, without that diode there to constantly power the Critical
Systems Bus from the Avionics Bus, I'd end up rebooting my equipment
during the switch change between running on the E-Bus during engine
cranking, to switching back to being fed from the Avionics Bus for
running. The nice thing about the arrangement though is that my
critical systems can stay up all of the time, as long as the E-Bus
Alternate Feed switch is on, and it doesn't matter what happens during
engine cranking. Once the engine is running, I can shut the Alternate
Feed switch off and let it be fed direct from the Avionics Bus.

Pay close attention to the diodes too. Be aware that diodes will
cause a drop in supply voltage between .3 volts and .7 volts depending
on the diode used. I had one of the regular E-Bus diodes in the system
already, but for the other I went with one from Perihelion Design that
is a low-dropping schottky diode. In order for your Aux Battery to get
a full charge, you want to keep that drop as low as possible. I'd use 2
schottky's if I did it all over again. They're harder to mount but be
creative and they can work great. The system works absolutely
fantastically the way it is laid out, which took a lot of thinking.

Here's my start procedure, for the interested, just to see the switch
dance isn't too bad:

It looks like a long list, but think through it and sit in the cockpit
and flip switches while you read the checklist and you'll see that
you're just basically turning a couple things on before cranking, and
off after the engine is running and Avionics Master is on. It takes a
lot of looking at someone elses diagram before it sinks in well, but
studying it well should reveal how it works with switches in certain
positions. Hope this answers questions. I haven't found anything but
good in the design for the most part, but that's my personal opinion
and you may have vastly different needs. Just make sure to think about
what you want out of your system well before you lay out the power
diagram. You don't know ho many people come and ask what I did, only
AFTER they find they are not satisfied with how their intial layout is
working in practice.